Nitrite is the product of the decomposition of ammonia by the nitrifying bacteria present in the filter; since it is highly toxic, it is essential that its level in water is zero.
An optimal value of NO3 in all types of water is simply 0 mg / l. When nitrite is 0.5 mg / l or higher, it means that there are not enough bacteria present and therefore it is advisable to add a bacterial activator.